One of the major challenges of the beef industry is to provide a consistently tender product for consumers. Tenderness has been identified as the single most important factor affecting consumers' satisfaction and perception of taste (Morgan et al., 1991; and Savell et al., 1987 and 1991).
As consumers have become more conscious of the nutritional content of the foods they eat, the beef industry has been striving to produce a leaner meat product to satisfy the demands of consumers for low-fat beef. However, studies indicate that this lowering of the fat content of beef has been made at the sacrifice of taste. It has been suggested that, as the amount of subcutaneous fat in beef is reduced, intermuscular and intramuscular fat are lowered, causing an increased variation in meat palatability. Smith, G. C. et al. 1987. "Relationship of USDA quality grades to palatability of cooked beef." J. Food Qual. 10:269. Thus, tenderness, the sensory trait most affecting consumer acceptance of beef, is unacceptably inconsistent. Morgan, J. B., et al. 1991, "Using calcium chloride injection to improve tenderness of beef from mature cows," J. Animal Sci. 69:4469. In 1990, the National Beef Tenderness Survey indicated that over 20% of rib steaks, 40% of chuck steaks/roasts, and 60% of round steaks/roasts were slightly tough or tougher (Morgan et al., 1991).
Means for improving the tenderness of meat has included calcium chloride injection or infusion in prerigor meat. See e.g. Morgan et al., 1991. Similarly, postrigor injection of CaCl.sub.2 has been used as another means for improving meat tenderness. Kerth, C. R. 1995. Improvement of beef tenderness and quality traits with calcium chloride injection in beef loins 48 hours postmortem. J. Anim. Sci. 73:750-756. The objective of the administration of exogenous CaCl.sub.2 is to activate .mu. and m-calpain, which are calcium dependent proteases (Goll et al., 1992; and Koohmaraie, 1992).
Active calpains degrade muscle proteins during postmortem aging of beef. The degradation of muscle proteins, especially titin and nebulin, has been associated with improved beef tenderness (Huff-Lonergan et al., 1995). These proteins influence meat tenderness due to their unique large size, structural properties, position in the myofibril, interactions with other key proteins in the sarcomere, and their purported roles in myofibril integrity (Robson et al., 1991; and Huff-Lonergan et al., 1996).
Through the action of calcium-dependent ATPase, the cytosolic calcium concentration of muscle cells is maintained at about 0.1 .mu.M, which is less than the Km values of the two calpains or calcium-activated proteases in skeletal muscles. The concentration of calcium in blood is maintained by a group of regulatory hormones called parathyroid hormone (PTH), calcitonin, and 1,25-dihydroxyvitamin D. The latter hormone is synthesized from 25-hydroxyvitamin D in the kidneys, which is synthesized from vitamin D in the liver. The cumulative effect of actions of 1,25-dihydroxyvitamin D is the significant elevation of calcium concentration in blood and of the mitochondria. Furthermore, large intakes of vitamin D.sub.3 will increase blood calcium markedly via actions of additional 1,25-dihydroxyvitamin D (Horst and Littledike, 1979).
Recent research has indicated that skeletal muscle is an important target organ for vitamin D.sub.3 (Boland and Nemere, 1992; Matthews et al., 1977). Vitamin D.sub.3 has been shown to increase uptake and transport of calcium by the mitochondrial and sarcoplasmic membranes of rachitic chick skeletal muscle (Boland, 1985; Boland et al., 1983a; and Bygrave, 1978). 1,25-dihydroxyvitamin D and 25-hydroxyvitamin D have been reported to increase calcium uptake and cyclic AMP levels, and stimulate the phosphorylation of several membrane proteins including those which calmodulin binding capacity potentates in skeletal muscle (Boland and Boland, 1985; and Massheimer et al., 1990). Vitamin D.sub.3 has been shown to increase the phospholipid composition of the sarcoplasmic reticulum and mitochondria of rachitic chick skeletal muscle (Boland et al., 1983b). Thus, vitamin D.sub.3 increased calcium transportation has been related to the Ca.sup.2 + pump and not to increased permeability of the membrane by lowering the Km and increasing the Vmax (Boland et al., 1983a).
Skeletal muscles are made of numerous muscle fibers. Each muscle fiber contains several hundred to several thousand myofibrils. Each myofibril in turn has, lying side by side, about 1500 myosin filaments and 3000 actin filaments, which are large polymerized protein molecules that are responsible for muscle contraction. The actin filaments attach to a Z disc, which passes from myofibril to myofibril, attaching the myofibrils to each other all the way across the muscle fiber. The portion of a myofibril that lies between two successive Z discs is called a sarcomere.
Extracellular (blood), mitochondrial, and sarcoplasmic reticular calcium is in the millimolar range. The calcium-dependent ATPases maintain concentration of calcium in the cytosol of muscle cells so that it is about one ten-thousandth that of the surrounding compartments. Koohmaraie et al., 1987, found that it is .mu.-calpain, not m-calpain, which plays an important role in the fragmentation of myofibrils and in the improvement of meat tenderness resulting from postmortem storage at refrigerated temperatures. This is most likely due to the high concentration requirement of Ca.sup.2+ for m-calpain to be activated. It has been reported that if only one in every 250 sarcomeres of a myofibril was broken, a significant improvement in tenderness could occur (Marsh, 1981). If all of the .mu.-calpains present were maximally active almost all of the z-disks which lie at the ends of the sarcomeres would be degraded, a change that is not associated with postmortem aging (Koohmaraie et al., 1986; and Koohmaraie, 1984).
The instant inventors have found that feeding meat producing animals vitamin D, rather than treating the meat with calcium chloride postmortem, is a more effective means of tenderizing meat due to increased .mu.-calpain and possibly m-calpain activity in fragmenting myofibrils.
It is therefore a primary objective of the present invention to provide a novel means for improving tenderness of meat and meat products.
It is a further objective of the present invention to provide a means for improving tenderness of beef which does not affect the flavor of the beef.
It is still a further objective of the present invention to provide a means for improving tenderness of meat and meat products which is safe and cost effective.
The method and means of accomplishing each of the above objectives as well as others will become apparent from the detailed description of the invention which follows hereafter.